Technical Abstract:
The 'enemy release' theory predicts that parasites will be lost during invasion through selection against hosts with parasite'reduced fitness. The North American amphipod Crangonyx pseudogracilis has successfully invaded European waterways and we have recently described the vertically transmitted, sex ratio distorting microsporidium Fibrillanosema crangonycis from an invasive European population of the host. We have investigated the number of host and parasite introductions as well as host and parasite diversity. We are interested in the impact of the invasion event(s) on the genetic diversity of the host and the diversity of its microsporidian parasites. We predict that vertically transmitted microsporidia may be resilient to 'enemy release' pressures as vertical transmission is characterized by reduced impact on host fitness and is not dependent on invasive host density. Thus, we predict that vertically transmitted microsporidia will not be lost by invading hosts and may be selectively retained. We test this prediction by comparing the diversity of microsporidia in North America and invasive European crangonyctids. Additionally, vertically transmitted microsporidia may utilize sex-ratio distortion to increase transmission success. We predict that feminizing microsporidia may increase host establishment success by increasing the proportion of females in the host population and hence population growth rate. We test this prediction by comparing host genetic diversity of European and North American crangonyctids.
We characterize host and parasite genetic variation across sites in North America and Europe using molecular phylogeny. We demonstrate that North American crangonyctids are host to a diverse group of novel microsporidia. While the European population of C. pseudogracilis appears to be clonal with a single dominant vertically transmitted microsporidium, there is much greater heterogeneity in the microsporidia of North American crangonyctids. This may indicate that F. crangonycis has been selectively retained and that sex ratio distortion by the parasite may have aided host and parasite establishment.